QuickSort giving exception in thread “main” java.lang.StackOverflowError

Question

I have written quick sort code as below. The sort takes middle number as pivot:

import java.util.Arrays;

public class QuickSort {

    public static void main(String[] args) {

        int[] numbers = {3, 6, 9, 1, 34};

        int low = 0;

        int high = numbers.length - 1;

        quicksort(numbers, low, high);  

    }

    static void quicksort(int[] arr, int low, int high) {

        int i = low;
        int j = high;

        int middle = arr[(low + high) / 2];

        while(i < j) {      
            while(arr[i] < middle ) {
                i++;
            }

            while(arr[j] > middle) {
                j--;
            }

            if( i <= j) {
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }


        }

        if(low < j) {
            quicksort(arr, low, j);         
        }

        if(i < high) {
            quicksort(arr, i, high);
        }

        System.out.println(Arrays.toString(arr));

    }

}

However, after running the code i am getting stackoverflow exception. It says Exception in thread "main" java.lang.StackOverflowError

Exception in thread "main" java.lang.StackOverflowError
    at QuickSort.quicksort(QuickSort.java:47)
    at QuickSort.quicksort(QuickSort.java:47)
    at QuickSort.quicksort(QuickSort.java:47)
    at QuickSort.quicksort(QuickSort.java:47)
    at QuickSort.quicksort(QuickSort.java:47)
    at QuickSort.quicksort(QuickSort.java:47)
    at QuickSort.quicksort(QuickSort.java:47)

Please help me find out what might be wrong in running the above code.

Answer 1

The value of i and j needs to be incremented and decremented respectively. Please see below the working code:

import java.util.Arrays;

public class QuickSort {

    public static void main(String[] args) {

        int[] numbers = {3, 9, 6, 1, 34};

        int low = 0;

        int high = numbers.length - 1;

        quicksort(numbers, low, high);  

    }

    static void quicksort(int[] arr, int low, int high) {

        int i = low;
        int j = high;

        int middle = arr[(low + high) / 2];

        while(i <= j) {      
            while(arr[i] < middle ) {
                i++;
            }

            while(arr[j] > middle) {
                j--;
            }

            if( i <= j) {
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;

                i++;
                j--;
            }


        }

        if(low < j) {
            quicksort(arr, low, j);         
        }

        if(i < high) {
            quicksort(arr, i, high);
        }

        System.out.println(Arrays.toString(arr));

    }

}

Hope this helps!

Answer 2

You can look at this link for better understanding of quick sort,how its work.

public static void quickSort(int[] arr, int low, int high) { if (arr == null || arr.length == 0) return;

        if (low >= high)
            return;


        int middle = low + (high - low) / 2;
        int pivot = arr[middle];


        int i = low, j = high;
        while (i <= j) {
            while (arr[i] < pivot) {
                i++;
            }

            while (arr[j] > pivot) {
                j--;
            }

            if (i <= j) {
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
                i++;
                j--;
            }
        }

        // recursively sort two sub parts
        if (low < j)
            quickSort(arr, low, j);

        if (high > i)
            quickSort(arr, i, high);
    }